Reboiler heat control responsive to overhead vapor content



United States Patent Office 3,255,595 Patented June 14, 1966 This invention relates to the automatic control of vapor pressure in an LPG product and more particularly, it concerns the automatic control of vapor pressure in an LPG product consisting largely of propane resulting as a byproduct of the stabilization of gasoline.

It is common practice in the petroleum industry to remove propane from the volatile overhead components in the stabilization of gasoline and sell the propane in liquid form commercially as LPG (liquefied petroleum gas). The vapor pressure of propane (186 p.s.i.a. at 100 F.) is less than the maximum vapor pressure for commercial LPG permitted by ASTM standards which is 210 p.s.i.g. at 100 F. Accordingly, it is desirable from the standpoint of economics to achieve an LPG product having a vapor pressure which approaches without exceeding the maximum ASTM standards. To this end, certain amounts of more volatile, less valuable hydrocarbons such as ethane are permitted to be mixed with propane to elevate the vapor pressure of the propane to a target pressure approaching the ASTM specifications.

The gasoline stabilization plant conventionally employs a stabilizer, a debutanizer and a depropanizer in series; the overhead product from the depropanizer being the LPG to which maximum vapor pressure specifications apply. In the past the vapor pressure of the LPG was controlled by manually sampling the LPG product, determining the vapor pressure of the sample in the refinery laboratory, and then adjusting the stabilizer reboiler temperature in accordance with the results of the laboratory determination to either increase or decrease the volatile components of the stabilizer bottoms product. As a result, a considerable problem was presented principally because of the amount of time which lapsed between the time at which the sample was taken, the time at which the compensating adjustment was made and the time at which the effects of the adjustment were realized. For example, it was not uncommon to incur a delay of five hours between the time the sample is taken in the plant and the time at which the results of the laboratory determination of vapor pressure was available. Then, because of the long transportation lag through the processing plant,

2 The control feature of the present invention is effected automatically by connecting a tap in the debutanizer overhead line to a chromatograph which generates a signal an additional four to six hour lapse was incurred before the effects of the change or adjustment were appreciated. Accordingly, the .stabilizer reboiler temperature, at best, could be adjusted only to produce a broad range of vapor pressures in the LPG product, the upper limit of which could not exceed the maximum permitted by specification. As a result, the average LPG product vapor pressure was substantially below the maximum permitted, thereby giving rise to a reduction in profits gained from the LPG product.

In accordance with the present invention, the vapor pressure of the LPG product in the depropanizer overhead of a stabilizing plant having a stabilizer or de-et-hanizer, debutanizer, and depropanizer in series, is accurately controlled by regulating the stabilizer reboiler temperature in accordance with the ethane content of the debutanizer overhead. This control arrangement operates effectively because first it. has been found that all of the ethane in the debutanizer overhead passes to the LPG product and secondly because the amount of ethane in the LPG product contributes significantly to the LPG vapor pressure.

proportional to the ethane content in debutanizer overhead samples taken automatically in predetermined time intervals. The chromatograph output signal is transmitted to a pneumatic controller which in turn effects an output signal for readjusting the temperature set point on the stabilizer reboiler temperature controller. In this manner, the vapor pressure of the LPG product is under ahnost continuous observation and further, the control point in the processing system is advanced well ahead of the LPG product so that the time lag previously experienced between measuring the vapor pressure and achieving an adjusted vapor pressure to meet specifications is materially reduced.

Accordingly, a principal object of this invention is to provide a gasoline stabilizing plant of the type having a stabilizer, debutanizer and depropanizer in series with a control arrangement by which the vapor pressure of the LPG product from the depropanizer overhead is accurately and constantly maintained within a narrow range commensurate with vapor pressure specifications.

Another object of this invention is that of providing an LPG vapor pressure control apparatus in a gasoline stabilizing plant of the type referred to which is wholly automatic and thereby requires a minimum of attention by plant personnel.

A further object of this invention is to provide a control arrangement of the type referred to by which the time lag between the obtaining of a sample by which LPG vapor pressure may be determined and the making of adjustments for varying the LPG product is reduced to a minimum.

Still other objects and further scope of applicability of the present invention will be realized from the detailed description given hereinafter in conjunction with the attached drawing, which illustrates diagrammatically the control apparatus of the present invention in a conventional gasoline stabilizing plant.

Referring now to the drawing, conventional gasoline stabilizing apparatus is shown to include a stabilizer or de-ethanizer 10 having a supply pipe 12 for the introduction of an unstable gasoline feed, a vapor line 14 for removing overhead products and a bottoms pipe 16. The stabilizer operates in a well known manner as a deethanizer to pass an overhead consisting mostly of ethane and methane through the vapor line 14 though it will be appreciated that small amounts of propane, propylene and butane in decreasing quantities respectively will also pass through the vapor line. Correspondingly, the bottoms product passed through the pipe 16 will be mostly propane and heavier. The fractions passing through the overhead and bottoms pipes, respectively, are determined primarily byheat supplied to the stabilizer and for this purpose a reboiler 18 is provided to which a suitable heating medium such as steam is supplied through a pipe 20 having a control valve 22 therein.- Thus, the quantity of lighter hydrocarbons including ethane passing with the bottoms product through the pipe 16 will be increased by lowering thetemperature of the reboiler and correspondingly, decreased by increasing the reboiler temperature.

The bottoms product from the stabilizer or de-ethanizer 10 is fed to a de-butanizer 24 having a bottoms pipe 26 through which stabilized gasoline is passed and an overhead 28 for butane and lighter hydrocarbons. The debutanizer overhead products are then passed through a series of treaters 30 in which various impurities such as I H S, mercaptanes and the like are removed. After passing through the treaters 30, the debutanizer overhead is fed to a depropanizer 32 having a butane bottoms pipe 34 and an-overhead pipe for propane and any lighter hydrocarbons remaining which are passed to storage designated at 38 as the LPG by-product.

The LPG product from the depropanizer overhead will, in this instance, be substantially all propane with small amounts of ethane. Also, the LPG will contain some propylene to be removed prior to marketing. However, because the vapor pressures of propane and propylene are approximately the same while the vapor pressure of ethane is substantially higher than either propane and propylene, the difference between the vapor pressure of the LPG and that of propane and propylene will be principally as a result of the ethane content. Also, it has been found that all of the ethane in the debutanizer overhead passes to the LPG product from the depropanizer overhead. The ethane content in the LPG by-product and therefore the vapor pressure of the LPG, may be controlled by varying the reboiler temperature in the stabilizer 10.

In accordance with the present invention, the vapor pressure of the LPG by-product is controlled by measuring the ethane content in the debutanizer overhead and adjusting the temperature of the reboiler 18 in the stabilizer in increments corresponding to the difference between the debutanizer overhead ethane content and the ethane content in the LPG product which will raise the vapor pressure thereof to a target point approximating the maximum permissible vapor pressure. To this end, a chromatograph 40 is connected to the debutanizer overhead pipe 28 by way of a tap 42. The chromatgraph 40 is, in itself, well known in the .art and may be arranged to sample the debutanizer overhead product periodically (e.g. every 12 minutes) and generate an output signal corresponding to the ethane content therein. The output signal from the chromatgraph 40 is transmitted to a percent ethane recorder-controller 44 having a manually adjustable set point illustrated diagrammatically by an arrow 46. The percent ethane recorder-controller is also, in itself, a conventional pneumatic controller which operates to sense the difference between the manually adjusted percent ethane set point and the output signal corresponding to the ethane content in the debutanizer overhead 28 transmitted from the chromatograph 40. The output of the recorder-controller 44 is then transmitted to the temperature set point illustrated diagrammatically by the arrow 48 of a reboiler temperature controller 50 which in min controls reboiler heat by regulating the valve 22.

The control system established by the percent ethane recorder-controller 44 and the reboiler temperature controller 50 is commonly known as a cascade control because of the manner in which the output signal from a primary controller, in this instance the percent ethane recorder-controller 44, resets the set point of a secondary controller, in this case the reboiler temperature controller 50. Various forms of cascade controls are described in Honeywell Technical Data Bulletin No. TX 119-1.

In operation, the set point on the percent ethane recorder-controller 44 is manually adjusted to achieve a reboiler temperature by way of the reboiler temperature controller 50, at which the ethane content of the stabilizer bottoms product and thus the ethane content in the debutanizer overhead 28 and depropanizer overhead 36 will achieve the desired target vapor pressure in the LPG. It will be appreciated, however, that: variations in the ethane content in the debutanizer overhead 28 and thus in the LPG will occur even though the reboiler temperature is maintained, either because of variations in the stabilizer feed or because of variations in the operation of the debutanizer 24. The occurrence of such variations, however, will be almost immediately detected by the chromatograph 40 and the ethane recorder-controller 44. -In other words, should the ethane content measured by the chromatograph exceed the percent ethane set point, the controller 44 will reset the controller 50 by an amount proportional to the excess 4 to raise the temperature of the reboiler 18. Conversely, if the ethane content in the debutanizer overhead 28 falls below the percent ethane set point, the output of the ethane recorder-controller will reflect the deficiency and automatically adjust the set point of the temperature controller 50 to decrease the reboiler temperature.

Thus it will be seen that by this invention there is provided an extremely effective method and apparatus for controlling the vapor pressure of the LPG product issuing from the depropanizer overhead and by which the above-mentioned objectives are completely fulfilled. Not only does the automatic control apparatus result in a considerable savings of time as compared with previously employed techniques, but also the accuracy of vapor pressure control is greatly enhanced. The greater accuracy results to some extent by the almost continuous determination of the ethane content ultimately passed to the LPG product and corresponding substantially continuous control over the reboiler temperature in the stabilizer. A factor which contributes equally to the accuracy of control, however, is the sampling of the ethane content in the debutanizer overhead rather than directly in the LPG product, since the substantial time lag involved in passage of the deubtanizer overhead through the treaters 30 and depropanizer is completely avoided. Accordingly, a much closer correlation between the control point and the sample point is possible with the present invention than was possible previously.

It will be apparent to those skilled in the art that the present invention may take numerous forms and find application in ditferent specific environments than the one described above. Accordingly, it is to be distinctly understood that the foregoing description is illustrative only, not limiting, and that the true spirit and scope of the invention is to be determined by the appended claims.

The invention claimed is:

1. Apparatus for the production of LPG as a byproduct from the stabilization of gasoline, said apparatus comprising in combination: a stabilizer having a reboiler, an overhead pipe for light hydrocarbons and a bottoms pipe for heavier components including propane, butane and some ethane; a debutanizer having an overhead pipe for butane and lighter components; a depropanizer having an overhead outlet for the LPG product consisting essentially of propane and some ethane, said stabilizer, said debutanizer and said depropanizer being in series flow relation; and means for controlling the vapor pressure of the LPG product, said means comprising signal generating means for producing a signal corresponding to the ethane content in the debutanizer overhead pipe, and means for regulating the temperature of said reboiler. in response to said signal.

2. In a gasoline stabilization plant including a deethanizer having a reboiler, a debutanizer and a depropanizer in series, apparatus for controlling the vapor pressure of an LPG product from the depropanizer overhead, said apparatus comprising; means for measuring the ethane content of the debutanizer overhead product and for generating a signal corresponding to said ethane content; and means responsive to said signal for controlling the temperature of said reboiler.

3. In a gasoline stabilization plant including a stabilizer, a debutanizer and a depropanizer in series, apparatus for controlling the vapor pressure of an LPG product from the depropanizer overhead, said apparatus comprising: means for measuring the ethane content in the debutanizer overhead and for generating a signal corresponding to said ethane content, a reboiler temperature controller for the stabilizer, said controller having a temperature set point, and means for readjusting the temperature set point automatically in response to variations between ethane content indicated by said signal and the ethane content in the LPG product establishing a desired vapor pressure.

4. The apparatus recited in claim 3 in which said measuring means is a chromatograph.

5. In a gasoline stabilization plant of the type having.

a de-ethanizer, debutanizer and depropanizer in series, apparatus for controlling the vapor pressure of an LPG product from the depropanizer overhead, said apparatus comprising: a chromatograph connected to the debutanizer overhead for generating a signal corresponding to the ethane content therein; a percent ethane controller connected to said chromatograph said controller having a manually adjustable set point and developing an output signal corresponding to the difference between the set point and the ethane content indicated by said chromatograph; and a reboiler temperature controller for the deethanizer, said temperature controller having a temperature set point adjustable by the output signal from said percent ethane controller.

6. The method of controlling vapor pressure of a propane LPG product from the depropanizer overhead in a gasoline stabilizing plant of the type having a de-ethanizer, debutanizer and depropanizer in series, comprising: regulating the temperature of the de-ethanizer in accordance with the ethane content in the debutanizer overhead.

7. The method of controlling vapor pressure of a pressure of a propane LPG product from the depropanizer overhead in a gasoline stabilizing plant of the type including a de-ethanizer, debutanizer and depropanizer in series, comprising the steps of establishing a reboiler temperature for the de-ethanizer to pass a predetermined amount of ethane to the de-ethanizer bottoms product, periodically measuring the ethane content in the debutanizer overhead product, and adjusting the de-ethanizer reboiler temperature to a point where the ethane content 8. Apparatus for the production of LPG as a by-product from the stabilization of gasoline, said apparatus comprising in combination a stabilizer having a reboiler, an

overhead pipe for light hydrocarbons and a bottoms pipe References Cited by the Examiner UNITED STATES PATENTS 2,096,700 10/1937 Smith 196132 2,355,589 8/1944 Brandt 6237 2,378,116 6/1945 Whaley 196132 2,500,353 3/ 1950 Gantt 62-28 2,627,318 2/1953 Swerdloflf 62-23 2,764,536 9/1956 Hutchins 62-37 2,774,715 12/1956 Gilmore 196--132 X 2,835,116 5/1958 Miller 6237 2,952,983 9/ 1960 Gilmore 6224 3,009,864 11/1961 Webb 62-21 3,028,332 4/ 1962 Forbes 62--23 3,111,460 11/1963 Orr.

NORMAN YUDKOFF, Primary Examiner. 

6. THE METHOD OF CONTROLLING VAPOR PRESSURE OF A PROPANE LPG PRODUCT FROM THE DEPROPANIZER OVERHEAD IN A GASOLINE STABILIZING PLANT OF THE TYPE HAVING A DE-ETHANIZER, DEBUTANIZER AND DEPROPANIZER IN SERIES, COMPRISING: REGULATING THE TEMPERATURE OF THE DE-ETHANIZER IN ACCORDANCE WITH THE ETHANE CONTENT IN THE DEBUTANIZER OVERHEAD. 